Part I An introduction to Thermochemistry Thermodynamics Part I An introduction to Thermochemistry

Thermochemistry The study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling. Generally is concerned with energy exchange accompanying transformations, such as mixing, phase transitions & chemical reactions Includes calculations of such quantities as the heat capacity, heats of combustion, heats of formation, heats of fusion, enthalpy, and Gibbs Free Energy.

Thermochemistry H-H bond C-C bond C-H bond 436 kJ/mole 332 kJ/mole Energy (heat) is involved in all chemical reactions Energy is stored in chemical bonds Energy is released or absorbed when bonds break or form in a chemical reaction H-H bond 436 kJ/mole C-C bond 332 kJ/mole C-H bond 418 kJ/mole

Enthalpy (∆ H) ∆ H = Enthalpy is equal to the change in the internal energy of the system, plus the work that the system has done on its surroundings change in enthalpy is the heat/energy absorbed or produced by a chemical reaction. ∆ H = Enthalpy (heat of a reaction) Endothermic vs. Exothermic Reactions

Endothermic Reactions in “thermic” heat energy is absorbed (put into) a chemical reaction energy/heat is a reactant ∆ H has a positive value (+ ∆ H) Reactants + energy → Products Reactants + ∆ H → Products + Reactants → Products ∆ H = ##

ex: CO2 + 2 H2O + 890 kJ/mole → CH4 + 2 O2

analysis of reactants, products & energy Stored energy Stability less more more stable less stable Energy must be put into reactants to cause the chemical reaction ∆ H = + 890 kJ/mole

- Exothermic Reactions Reactants → Products + energy out “thermic” heat energy is released in a chemical reaction energy/heat is on the products side ∆ H has a negative value (- ∆ H) Reactants → Products + energy Reactants → Products + ∆ H - Reactants → Products ∆ H = ##

ex: 2 Ag + 2 HCN → 2 AgCN + H2 + 594 kJ/mole

analysis of reactants, products & energy Stored energy Stability more less less stable more stable unstable reactants release their energy ∆ H = - 594 kJ/mole

Bomb Calorimeter device used to measure the heat of chemical reaction Exothermic reaction heats up water Temp of water decreases in Endothermic Reaction Specific Heat Capacity of Water = 4.184 J/gC

Specific Heat Capacity the measure of heat or thermal energy required to increase the temperature of a unit quantity of a substance by one unit. Energy needed to raise 1 gram of a substance 1 degree Celsius Water = 4.184 J/gC Aluminum = 0.897 J/gC Methane = 2.20 J/gC Tungsten = 0.132 J/gC

Calculating Enthalpy Changes Experimentally using a calorimetry ∆ H = m x ∆ T x Cp mass x temp change x specific heat

∆ H = 63,094.72 J ∆ H = m x ∆ T x Cp ∆ H = - 63.09 kJ x x problem #1 Calculate the heat of reaction if 412 grams of NaOH heats 520 grams of water from 22 C to 51 C ∆ H = m x ∆ T x Cp x x 520 g 29 C 4.184 J/gC ∆ H = 63,094.72 J ∆ H = - 63.09 kJ

∆ H = 110,546.28 J ∆ H = m x ∆ T x Cp ∆ H = + 110.55 kJ x x problem #2 A 1,560 gram cube of Aluminum has an initial temperature of 98 C and is cooled to 19 C. Determine the enthaply change. ∆ H = m x ∆ T x Cp x x 1,560 g 79 C 0.897 J/gC ∆ H = 110,546.28 J ∆ H = + 110.55 kJ